Method and device for connecting an optical element to a frame

ABSTRACT

A method and a device for the material-fit connection of an optical element to a frame are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims benefit under 35 USC120 to, international application PCT/EP2007/009177, filed Oct. 23,2007, which claims benefit of German Application No. 10 2006 050 653.7,filed Oct. 24, 2006 and U.S. Ser. No. 60/862,617, filed Oct. 24, 2006.International application PCT/EP2007/009177 is hereby incorporated byreference.

FIELD

The disclosure relates to a method and a device for the material-fitconnection of an optical element to a frame.

BACKGROUND

Methods for connecting optical elements to frames are known.

SUMMARY

In some embodiments, the disclosure provides a method and a device, bywhich an optical element and a frame can be connected togetherharmlessly and with reduced drift of the two assembly partners.

In certain embodiments, the disclosure provides a material-fitconnection of an optical element to a frame. A connection medium ismelted locally on at least one connection position between the opticalelement and the frame. The optical element and the frame is inmechanical contact with one another at each instant in the connectionprocess on at least three connection positions through non-moltenregions of the connection medium. In other words, the optical elementrests on the frame during the connection process on at least threepositions through non-molten parts of the connection medium which isprovided. The effect achieved by this is that the orientation of theoptical element in relation to the frame is defined at each instant inthe connection process, since floating of the optical element on a fullymolten region of the connection medium cannot take place. The non-moltenregions of the connection medium are therefore are used during theconnection process as positioning aids in the manner of a holdingtemplate by which the relative position of the two assembly partners,i.e. the optical element and frame, is constantly defiant. Thedisclosure can therefore allow highly accurate and reproducible assemblyof the optical element and the frame.

For example, the optical element and the frame may be connected togetheron at least four connection positions, in a first step energy inputbeing carried out on at least three of the at least four connectionpositions so that a non-molten region of the connection medium remainsduring the energy input on the relevant connection position, throughwhich the optical element is in mechanical contact with the frame. Thenon-molten regions of the connection medium are thus used as supportsfor the optical element relative to the frame; the described partialmelting of the connection medium on the first at least three connectionpositions may also be referred to as a fixing process or as attachment.Subsequently, in a further step, the optical element and the frame areconnected together by fully melting the connection medium on theindividual connection positions in succession.

If for example a circular optical element is selected as the opticalelement, for example a lens, then the three positions which aresubjected to the fixing process in the first step may be arranged at anangular spacing of about 120° along the circumference of the lens, so asto obtain particularly stable mounting and positioning of the lensrelative to that frame during the fixing process and the subsequentconnection process.

The connection medium may in particular designed as a solder preformwith a thickness of about 50 μm to 100 μm and lateral dimensions ofabout 5×5 mm; it is however also conceivable for the connection mediumto be vapor deposited, sputtered or electrolytically applied onto theoptical element and/or the frame as a coating in the region of theconnection position.

A laser beam with a power of about 100 watts may advantageously be usedfor melting the connection medium. In this case, the fixing process ofthe first step may be achieved by illuminating the connection positionfor about 50 milliseconds with a laser spot with a size of 1 mm².

For example, tin-based solders such as SnAg4, tin-lead solders, solderscontaining bismuth such as BiSn42, solders containing indium, solderscontaining antimony, gold-based solders and solders containing gold,silver or palladium are suitable as the connection medium.

The full melting of the connection medium in the subsequent step may,for example, be achieved by employing a controlled soldering processwhile monitoring the temperature of the connection position—inparticular by using a pyrometer.

In particular lenses or mirrors, for example of a projection exposureapparatus for semiconductor lithography, may be envisaged as opticalelements.

With the method described above, it is possible in particular to producea projection exposure apparatus for semiconductor lithography in whichthere is an optical element arranged in a frame with a material fit viaa connection medium, the optical element being in mechanical contactwith the frame only through the connection medium. In other words, theoptical element does not rest directly on the material of the frame atany position.

This type of framing technique is suitable in particular for thin-edgedlenses for example with an edge thickness of less than 1 cm, but alsofor mechanically manipulable lenses or even a first or last lens in aprojection objective of the projection exposure apparatus in the beamdirection. In the case of an EUV projection exposure apparatus, theoptical element may also be a non-mechanically manipulated mirror of theapparatus; such mirrors are conventionally exposed to high thermal loadsin EUV projection exposure apparatus.

In some embodiments, the optical element is connected to the frame on atleast one connection position via a connection medium containing atleast one metallic component. The optical element and the frame aremutually positioned before the connection in order to avoid relativemovements during the connection, and the positioning being carried outindirectly or directly by at least one bearing element which is designedas part of the frame. This measure can prevent a liquid layer of anadhesive, cement, liquid metal, solder or the like from being formedbetween the optical element and the frame during the connection process,on which the optical element floats and therefore changes its positionrelative to the frame. The optical element therefore remains connectedto the frame through a rigid support during the connection process, andthe selected relative position is reliably maintained.

The frame and the at least one bearing element may, for example, bedesigned integrally; an option includes connecting the frame and the atleast one bearing element releasably, for example connecting themtogether by screwing. The effect of the latter procedure is that thebearing element can be removed from the frame after the connectionprocess. The advantages of this procedure are in particular that, afterthe at least one bearing element has been removed from the frame, thenumber of contact positions of the optical element with the frame isreduced and the risk of for example thermally induced stresses in theoptical element during its use is therefore substantially reduced.Another advantage of the method is that the installation space desiredfor the combination of the frame and optical element is considerablyreduced, which is very important for installation space-criticalapplications, for example for projection exposure apparatus and inparticular objectives for semiconductor lithography, in which theoptical elements framed by the method may advantageously be employed.

It has been found advantageous for the optical element to rest on the atleast one bearing element on at least three points. Mounting on threepoints satisfies the desired properties for stable mounting of theoptical element in the frame. Mounting on an edge may also be envisaged.

An advantageous method for material-fit connection of the opticalelement to the frame consists in employing a soldering method, inparticular by using a flux-free solder alloy. In some embodiments, thesolder may be applied beforehand onto the frame and/or the opticalelement as a layer. For example PVD, CVD, electrolytic or chemicalmethods may be envisaged as coating methods. The solder may likewise beapplied as a molten coating. In particular tin-based solders (forexample SnAg4), tin-lead solders, solders containing bismuth such asBiSn42, solders containing indium, solders containing antimony,gold-based solders and solders containing gold, silver, palladium may beenvisaged.

In certain embodiments the solder may be applied as a preform on theframe and/or on the optical element before the connection process. Thepreform may advantageously be produced by stamping it from a foil; inthis case, suitable shaping of the preform and frame or the opticalelement can achieve the additional advantageous effect that the preformis aligned straightforwardly on the frame or the optical element. Thismay for example be ensured by providing mutually corresponding positionmarkers on the preform and frame, or the optical element.

Particularly when using a preform stamped from a foil, it has been foundadvantageous to configure the frame in the region of the connectionposition so that it follows the geometrical shape of the opticalelement. Assuming a constant thickness of the preform, this provides agap with an approximately constant thickness between the preform and theoptical element. The thickness of the gap may tend toward zero; in thiscase, however, it is usually desired to ensure that the optical elementrests on the at least one bearing element and not on the preform.

Optionally, a method may be envisaged in which the optical element restson the preform at the bearing points; the mutual position of the opticalelement and the frame is in this case defined by the opticalelement—preform—bearing element arrangement. In this case as well, it ispossible to keep the mutual position of the elements constant during thesoldering process. To this end, it is merely desired to ensure that thepreform is only melted locally during the soldering process, i.e.specifically not in the region of the bearing point. For example asolder platelet, which is melted only pointwise or in regions during thesoldering process, may be used as a preform. This can reliably preventthe optical element from floating on the preform during the solderingprocess. This variant has the particular advantage that comparativelysimple preforms can be used.

In the variant explained above, as already mentioned, the position ofthe optical element relative to the bearing element is also dictatedessentially by the platelet thickness. If the platelet is only meltedpointwise or in regions, for example by a laser beam, then theorientation of the optical element relative to the bearing element isco-defined by the platelet thickness. In this case as well, thepositioning of the optical element relative to the bearing element isdefined through a form fit, specifically by an indirect form fit due tothe platelet. Only when the platelet is melted throughout the bearingregion between the optical element and the bearing element does thepositioning of the optical element relative to the bearing element takeplace no longer by a form fit but by material fit, the “positioningaccuracy” being determined essentially by the thickness of the platelet.

In order to improve the strength of the solder connection, it has provenexpedient to apply a solderable layer system on the optical element,which may in particular be produced as a metallization layer in theregion of the connection position. Different variants may be envisagedin respect of the configuration of the metallization layer. One aspectin the configuration of the metallization layer is that the heating ofthe solder and the connection position on the holder and the opticalelement can be carried out via a laser beam, in particular by using anIR or UV-VIS laser. A first possibility consists in heating the solderthrough the optical element and the metallization layer. For thisprocedure, it is advantageous to configure the metallization layer notas a continuous layer of constant thickness, but to provide it withopenings, or not to bound to the metallization layer by sharp edges.This will ensure that a sufficiently large part of the energy of thelaser beam reaches the solder, in order to liquefy it.

In certain embodiments, the laser beam is reflected at least partiallyby the other side of the solderable layer system from the opticalelement, and in the reflected fraction heating the solder. Thisprocedure has the particular advantage that the absorption properties ofthe optical element do not have to be taken into account when selectingthe laser. It furthermore avoids damage of the optical element due tothe energy introduced into it by the laser beam being used.

In order to further increase the quality of the solder connection, theframe may be heated by an additional laser beam in the region of theconnection position. The first and additional laser beams may come fromthe same laser source, for example having been separated by a beamsplitter. Optionally, the additional laser beam may come from anadditional laser source; this provides the opportunity to select theadditional laser beam in accordance with the features of the frame, forexample its absorption properties.

The occurrence of intrinsic solder stresses or other stresses in theregion of the connection position may be achieved by reducing the laserpower in a controlled way after the soldering process per se, so thatthe energy input per unit time decays gradually. As an alternative or inaddition, the stresses may also be relaxed by subsequent heat treatmentof the arrangement.

In some embodiments, the at least one bearing element is in contact withthe optical element in the region of an optically active surface of theoptical element, and the connection position lies on an opticallyinactive surface of the optical element. An “optically active surface”is intended to mean a surface of the optical element which transmits orreceives light during use of the optical element. In the case of a lensas the optical element, the optically inactive surface is intended tomean the cylinder edge of the lens.

An advantageous possibility for connecting an optical element to a frameinvolves initially applying a metal layer onto the optical element andsubsequently connecting a region of the metal layer on the other sidefrom the optical element to the frame with a material fit.

In particular, a welding method may be used as the connection method.One advantage of this procedure is that only the part of the metal layernext to the frame is melted during the welding, while the part of themetal layer next to the optical element remains solid. The entirewelding process may take place within milliseconds—in which case thepart of the frame adjoining the metal layer likewise melts. For theduration of a laser pulse used for example in the scope of a laserwelding method, values of about 10 ms or about 0.5 s to 5 s areadvantageous. A permanent connection is therefore obtained within anextremely short time. The effect achieved by this is that the part ofthe layer in contact with the optical element is affected only minimallyby the welding process. The duration of the energy supply and thereforethe heat influence zone are very much smaller than for example comparedwith a laser soldering method, since for example a single short laserpulse is generally sufficient to join the assembly partners togetherwhen using a laser welding method. In particular, the temperature on thenonmetallic material of the optical element can therefore be kept muchlower than 100° C. (e.g., in the range of about 40° C.) especially sincethe weld point which is very much smaller compared with a solderposition solidifies much more rapidly than a solder connection; usually,the weld point will solidify more rapidly than the assembly partners canheat up. The metal layer with a thickness furthermore leads to effectivemechanical decoupling of the weld position from the optical element. Therisk of introducing both thermal and mechanical loads into the opticalelement is therefore effectively minimized. Furthermore, the increasedprocess reliability when using a welding method compared with asoldering method has a positive effect. In particular the desire toprevent the formation of an oxide layer for example by fluxes, whichconstantly exists in soldering methods, plays only a secondary role orno role at all in welding methods. A welding process furthermoreobviates the need to deposit an auxiliary material, for example solder,under cleanroom conditions on the connection position between the frameand the optical element, which is often problematic in terms ofhandling. In contrast to conventional soldering methods, the method canmake it possible to apply the metal layer onto the optical element longbefore the connection process; it therefore becomes an unlosablecomponent thereof—because it is firmly connected to the opticalelement—so that the process reliability and the manageability arefurther increased, for example in a cleanroom environment.

Furthermore, it is not necessary to coat the frame before the connectionoperation, so that the connection process is kept simple and thereforeeconomical.

The described welding process is also advantageous in terms of energy,since the energy is only introduced at the positions in the materialwhere it is needed.

Another advantage is that the risk of position changes during theconnection process is significantly reduced owing to the small volumewhich is melted. The position of the optical element relative to theframe can therefore be adjusted much more precisely, and also maintainedduring the connection process.

About 0.3-2 mm has proven to be an advantageous value for the thicknessof the metal layer; the diameter of the weld point may be selected in arange of about 0.1-1.5 mm.

Optionally, a brazing method can be used. A brazing method isdistinguished in that merely one of the parts to be connected is melted,for example only a part of the metal layer, while the frame is notmelted.

In some embodiments, the metal layer lies on an optically inactivesurface of the optical element. An “optically inactive surface” isintended to mean a surface of the optical element which transmits orreceives light during use of the optical element. In the case of a lensas the optical element, the optically inactive surface is intended tomean the cylinder edge of the lens. This measure has the advantage thatthe optical element can be processed further, in particular cleaned orcoated, even after the metal layer is applied.

A low-melting or soft alloy represents an advantageous selection for themetal layer. The effect achieved by this is that a lower energy input isdesired for connecting the optical element and the frame, so that thetemperatures occurring on the optical element can be kept low andstresses occurring can be relaxed by a plastic deformation of the softmetal layer during cooling. Another effect of suitable selection of thematerial of the metal layer is that the optical element can more easilybe removed again from the frame.

Another option for a method for the material-fit connection of anoptical element to a frame via a connection material consists in meltingthe connection material at least partially by short-term energy input.The energy input may be carried out by supplying a power of about 50 Wfor a period of less than 3 s. This procedure has the advantage that itleaves only to local heating and therefore to only local deformation,and therefore less change in the shape and position of the opticalelement and the frame. The energy input may be carried out byelectromagnetic radiation such as light, in particular lasers,microwaves, or inductively.

A soldering method may advantageously be used for connecting the twoassembly partners, in which case the solder used for the soldering maybe applied as a powder or paste before the connection process; anotherpossibility consists in applying the solder by a coating method, forexample a PVD or CVD method, a chemical or electrolytic method or as amolten coating.

It has been found particularly advantageous to apply the solder used forthe soldering as a preform before the connection process, for examplewith a thickness of about 100 μm. In particular, this measure achievesincreased position stability and retention holding during the connectionprocess compared with the prior art, as well as improved alignabilitybefore the connection process. This procedure also results in simpleapplication and fixing of the solder.

In some embodiments, the optical element resins are the bearing pointson the preform; the mutual position of the optical element and the frameis defined in this case by the optical element—preform—framearrangement. In this case as well, it is possible to keep the mutualposition of the elements constant during the soldering process. To thisend, it is merely desired to ensure that the preform is only meltedlocally during the soldering process, i.e. specifically not in theregion of the bearing point. This can reliably prevent the opticalelement from floating on the preform during the soldering process. Thisvariant has the particular advantage that comparatively simple preformscan be used.

In order to fix the preform on the frame, it has proven expedient forthe frame to include a bearing foot with a recess to receive thepreform; the recess may, in particular, be designed as a groove at leastapproximately in the form of a circle arc segment, or as an at leastapproximately cylindrical hole.

An advantageous selection for the placement of the connection positionis for it to lie on an optically inactive surface of the opticalelement. An “optically inactive surface” is intended to mean a surfaceof the optical element which transmits or receives light during use ofthe optical element. In the case of a lens as the optical element, theoptically inactive surface is intended to mean the cylinder edge of thelens. This measure has the advantage that the optical element can beprocessed further, in particular cleaned or coated, even after the metallayer is applied.

The quality of the connection position may be improved by the preform,and the region of the frame which is in contact with the preform, beingdesigned in such a way that the preform is pressed against the opticalelement by a spring force. The preform itself, the part of the framewhich is in contact with the preform, may be designed as a resilientelement.

In some embodiments, an optical element is connected to a frame via aholding element on at least one connection position consists in arecess, into which the holding element is introduced, being formed inthe optical element in the region of the connection position, andregions in the recess between the holding element and the opticalelement being filled by a filler material. The filler material may be amolten material which is introduced into the recess in the molten state,after the holding element has been introduced into the recess, into theregion between the holding element and the recess where it cools andsolidifies. In this way, at least a form-fit connection can be producedwithout precise preparation of the recess and the holding element beinginvolved. A material fit is not necessary in this first variant. Themechanical stability of the arrangement including the optical elementand the frame may be improved further by introducing a connectionmaterial, in particular a solder or an adhesive, into the recess in theoptical element in order to form a material-fit connection.

An advantageous soldering process may, in particular, be carried out byinitially applying a metallization layer applied via an electrolyticmethod on at least one of the assembly partners to be connected andsubsequently melting the metallization layer, the metallization layercontaining at least one admixture for improving the properties of thesolder position, for example increasing its mechanical stability. Theterm mechanical stability is intended to mean all properties of thesolder position which affect the robustness of the solder position inrelation to mechanical stresses such as tension, compression or shear.Furthermore, mechanical stability is also meant to be a smallinfluenceability of the mechanical properties of the solder position bytemperature variations. It is also conceivable to provide the twoassembly partners respectively with different electrolytically appliedmetallization layers before the actual soldering process. Electrolyticapplication of the metallization layer is not compulsory in this case;the solder may be applied in another way into the region of theconnection position, for example as a preform.

In this way, it is possible to counter effectively the conflict ofinterests that the low-melting metals used for the soldering arecomparatively soft, for example In or Sn, and therefore susceptible tocreep in the long term. On the other hand, the use of low-melting metalsis virtually inevitable since heating the sensitive optical componentsto temperatures higher than desired for a soldering process entailsconsiderable risks. The mechanical properties of the solder connectionmay be improved in particular by using dispersoids, in particularoxides, nitrides, borides or carbides; furthermore, a reduction in thethermal expansion coefficient of the solder connection may also beachieved if the dispersoid is a zerodur powder.

The formation of an intermetallic phase in the region of the solderposition, in particular by using phosphorus, or the use of microfibersor nanotubes, is also advantageous. In particular, the mechanicalproperties of carbon nanotubes may be used advantageously for this; theyhave a density of 1.3-1.4 g/cm³ and a tensile strength of 45 billionpascals, and therefore a ratio of tensile strength to density which isabout 135 higher than for example steel.

The above-described configurations of the connection position and thebearing elements may also be used advantageously in cases in which theconnection material is melted not by short-term but by prolonged energyinput. In particular, the described methods for the material-fitconnection of a frame and an optical element may also be combined withthe variants mentioned in detail above and below for configuration ofthe bearing elements and the connection positions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the disclosure, which differ in principle, willbe described below with the aid of the drawing, in which:

FIG. 1 shows a flow chart of a method;

FIG. 2 shows a framed optical element;

FIG. 3 shows mounting of the optical element;

FIG. 4 shows a device;

FIGS. 5 and 5 a shows a bearing element;

FIGS. 6 and 6 a shows a bearing element having a bore for receiving asolder preform;

FIG. 7 shows a bearing element having a vertical through-bore;

FIG. 8 shows a bearing element having a resilient element for fixing asolder preform;

FIG. 9 shows an essentially form-fit connection between the opticalelement and the frame;

FIG. 10 solder is as a preform;

FIG. 11 shows an optical element connected to a frame via a holdingelement;

FIG. 12 a-12 d shows a configuration of holding elements, and

FIG. 13 shows a projection exposure apparatus for semiconductorlithography.

DETAILED DESCRIPTION

FIG. 1 represents the disclosure by way of example with the aid of aflow chart. First, an optical element to be soldered into a frame isplaced into the frame; the frame has a multiplicity of so-called bearingfeet, on which the optical element is mounted. The bearing feet may bescrewed onto the frame or connected integrally with the frame. Solderpreforms, designed as platelets, are arranged between the bearing feetand the optical element. The platelets are about 50 to 100 μm thick andhave a lateral extent of about 5×5 mm. They are made of the materialsmentioned above. As an option in designing the solder preforms asplatelets, it is also possible to coat either the bearing feet or theoptical element with the connection material. This coating may forexample be carried out by vapor deposition, sputtering or anelectrolytic method. It is advantageous to coat the optical elementbeforehand on the positions to be brought in contact with the bearingfeet, since this gives better thermal contact during the subsequentsoldering process so that it is possible to operate with a shorter orweaker laser pulse. This means that heating of the bearing feet can beminimized as much as possible, and it reduces the risk of thermaldeformations of the bearing feet during the soldering process or due tothe soldering process. After the optical element has been placed ontothe bearing feet, temporary attachment of the optical element takesplace on three bearing feet. To this end, the preform is exposed to anNd:YAG laser with a power of about 100 watts through the opticalelement, and is thereby heated. A laser pulse with a length of about 50ms is sufficient for the fixing process; it is to be assumed that 40-60watts of the 100 watt laser output power input into the position to besoldered, owing to surface reflections and scattering. It isadvantageous for the three bearing feet, on which it is fixed, to beselected with an angular spacing of about 120° in order to ensuremaximally stable mounting of the optical element. Other distributions ofthe fixing positions may also be envisaged.

The actual soldering process subsequently takes place, i.e. theindividual solder positions are firmly soldered in succession. To thisend a controlled soldering process is employed, i.e. the surfacetemperature of the solder position is monitored by a pyrometer. Aheating process is initially carried out with a linearly increasinglaser power for about 0.5 s. As soon as the holding temperature of about200 to 250° C. (in the case of a bismuth-tin solder) is reached, thelaser power is kept constant for about 2 s. The laser power is thenreduced to 0 over a period of about 0.5 s. When using a silver-tinsolder, the holding temperature is about 300° C. In order to reinforcethe process, the solder may be supplemented with a flux which on the onehand reduces the surface tension of the liquid solder and thereforeimproves the mechanical contact, and on the other hand reduces thesurfaces oxidized during the soldering process. For example water-basedor alcohol-based, halogen-free fluxes may be suitable.

During the fixing process, the size of the laser spot being used is inthe range of about 1 mm², while a spot size which is matched to the sizeof the solder preform is selected for the actual soldering process. Thismay, for example, be achieved by using suitable optics and/or byselecting the distance from the light output surface of the laser to thesolder position. The described method is suitable in particular foroptical elements which are particularly stressed mechanically, thermallyor chemically, for example in a projection objective for semiconductorlithography. The described process may advantageously be employed inparticular for framing entry lenses or exit lenses of the projectionobjective, but also for mechanically manipulated optical elements. InEUV objectives, non-manipulated mirrors are particularly suitable forthe soldering process owing to the high heat loads to be expected.

FIG. 2 shows an optical element 1 framed in a frame 2. The opticalelement 1 is arranged in frame 2 so that it is mechanically connected tothe frame 2 at the connection positions 7 only through the connectionmedium 400. This type of framing technique is made possible by using themethod according to the disclosure, since it ensures precise solderingof the optical element 1 into a frame 2. A particular advantage of theframing technique represented in FIG. 2 is that the optical element 1 isconnected to the frame 2 through a uniform material system. In this way,inter alia, the occurrence of mechanical stresses due to temperaturevariations is effectively reduced, which is advantageous in particularfor use of the optical element framed in a projection exposure apparatusfor semiconductor lithography.

FIG. 3 shows details of the region of the connection position of theoptical element 1 and the frame 2. The optical element 1 rests on theframe 2 via the bearing element 3, which is produced in the presentexample as roof-like projection formed integrally with the frame 2. Thetwo preforms 4 a and 4 b are arranged on the frame 2 in the vicinity ofthe bearing element 3. The preforms 4 a and 4 b are matched to the formof the frame 2 in respect of their geometrical shape, for example beingstamped from a foil of suitable thickness. Since the frame 2 is alreadymatched to the geometrical shape of the optical element 1 in the regionof the connection position, a gap 5 a,b with a constant thickness isobtained between the optical element 1 and the preforms 4 a and 4 bowing to the constant thickness of the preforms 4 a and 4 b. Themetallization layers 6 a and 6 b are respectively applied onto thesurface regions of the optical element which correspond to the preforms4 a and 4 b. If the two preforms 4 a and 4 b are now heated, for exampleby the action of a laser beam, then they melt and contract owing to thesurface tension of the molten material, so that the gaps 5 a and 5 b areclosed and a material-fit connection is consequently produced betweenthe optical element 1 and the frame 2. The position of the opticalelement 1 relative to the frame 2 does not change because the positionis already fixed during the connection processed by the bearing element3, and the melting preforms 4 a and 4 b do not lead to floating andtherefore a position change of the optical element 1 relative to thatframe 2.

FIG. 4 shows an embodiment of the device employed for a method. In someembodiments, the optical element 1 designed as a lens is connected tothe frame 2 so that the connection position 7 of the optical element 1to the frame 2 lies on an optically inactive surface of the opticalelement 1. In the present case, this is achieved by the connectionposition 7 lying on the cylinder edge of the lens 1. The sphericalpreform 4 is fixed in the embodiment shown in FIG. 4 by its lyingbetween the cylinder edge of the lens 1 and a chamfered region of theframe 2 and being held in its position by the force of gravity. Thebearing element 3 is designed in the present case as part of anadditional holding element 2 a temporarily fixed on the frame 2 by ascrew connection 8. After the preform 4 has been liquefied by thefocused laser beam 9 and the material-fit connection between the opticalelement 1 and the frame 2 has been produced, the additional holdingelement 2 a can be removed from the frame 2 by releasing the screwconnection 8. The optical element 1 is then held in the frame 2 only bythe material-fit solder connection and the risk of additional, forexample thermally induced, stresses which are due to the holdingelements that are no longer required after the connection, iseffectively reduced by this solution.

FIG. 5 shows an advantageous embodiment of a bearing element 3. Thebearing element 3 has a slot-shaped indentation 10. Such an indentationmay, for example, be produced by initially machining the indentation 10from the solid material of the frame and subsequently excavating thebearing element 3 which is formed from the material of the frame 2, forexample by a sawing. The slot-shaped indentation 10 may be used toreceive a solder preform 4, as depicted as a cross-sectionalrepresentation in FIG. 5 a.

A configuration of a bearing element 3 is shown by FIG. 6. The bearingelement 3 is configured so that it has a so-called roof edge 11, onwhich the optical element 1 can be mounted. A cylindrical bore 12 mayextend over the roof edge 11. The bore 12 may, in particular, receive acorrespondingly shaped preform 4.

The effect of this measure is that the material-fit connection of theoptical element 1 to the bearing element 3 fulfils the function ofholding the optical element 1, the positioning of the optical element 1being carried out or defined by a form fit. In other words, there is atleast one bearing surface (or at least a bearing point or a bearingline) on the bearing element 3, which is in direct (immediate) contactwith the optical element 1 and therefore forms a pure form fit withrespect to the positioning of the optical element 1. In other regions ofthe bearing element 3, a material fit is additionally produced throughthe soldering via the preform 4. It should be taken into account herethat during production of the material-fit connection, local stresseswhich act on the optical element 1 may occur owing to some shrinkage ofthe solder when it solidifies. In order to absorb such stresses asuniformly as possible, the bearing element 3 optionally has asymmetrical roof edge 11 (symmetrical with respect to the material-fitconnection position). Depending on the size of the optical element 1 andthe size of the bearing element 3, the roof edge 11 may also be replacedby a flat support. The flat support is in this case optionally arrangedsymmetrically with respect to the position of the material-fitconnection and therefore symmetrically with respect to the preform 4.

Besides flat or linear support, it is also possible for the bearing totake place in an essentially point-like fashion, for example byproviding the edge (or surface) arranged to the left and right of thepreform 4 with additional bearing points.

FIG. 6 a shows a variant of the disclosure, slightly modified relativeto the embodiment represented in FIG. 6, in which the optical element 1rests on the bearing element 3 via a point-like support, the form fitbetween the optical element 1 and the bearing element 3 being producedby the point-like support. This may for example be achieved, as in FIG.6 a, by the preform 4 having a bore 19 through which a pin 20 connectedto the bearing element 3 projects, the pin being provided with a tip inthe direction of the optical element 1. The pin height is in this casematched to the height of the preform 4. The pin 20 optionally hasapproximately the same height as the preform 4, so that no solder wetsthe pin tip when the solder is melted, and so that a material-fitconnection is still produced between the solder and the optical element1, although the optical element 1 is mounted in a defined way in respectof its position by the pin 20.

As represented in FIG. 7, the bore 12 is configured so that it extendsin the vertical direction through the entire bearing element 3. Thisvariant has the advantage that the preform 4 can be inserted from belowthrough the bore 12 into the bearing element 3.

FIG. 8 represents another variant of the bearing element 3. The bearingelement 3 is divided into three subregions, of which the two outlyingsubregions 13 a and 13 b are designed in a way comparable to thatrepresented in FIGS. 6 and 7. The central subelement 14 is produced as aresilient tongue. This variant has the particular advantage that asolder preform 4 arranged on the resilient tongue 14 is pressed in thedirection of the optical element 1 by the spring force exerted by thetongue 14, and the material fit during the soldering is thereforeoptimally ensured. In this procedure, it is advantageous for the springforce of the resilient element 14 to be dimensioned so that the forceexerted on the optical element from below through the preform 4 is lessthan the weight force of the optical element 1 respectively acting atthe position of the bearing element 3. This ensures that the opticalelement 1 maintains its alignment, already finely adjusted before thesoldering process, during the soldering process.

In some configurations, the preform itself may also be designed as anelastic element. In this case, it is not necessary to equip the bearingelement itself with resilient elements.

FIG. 9 shows an embodiment in which the optical element 1 has a recess16 on its outside, into which a holding element 15 engages. The gapbetween the holding element 15 and the recess 16 may contain a fillermaterial 17 for further stabilization of the connection. For exampleadhesives, solder or cement may in particular be envisaged as a fillermaterial 17. The filler material 17 need not necessarily provide amaterial-fit connection—even a form-fit connection optimized by thefiller material 17 is sufficient for a mechanically stable connection.Since there cannot be mechanical contact between the optical element 1and the holding element 15 before the gap is filled with the fillermaterial 17, the connection between the optical element 1 and theholding element 15 can take place essentially stress-free.

FIGS. 9 a, 9 b and 9 c show various possibilities for the geometricalconfiguration of the recess 16 and the holding element 15; the holdingelement 15 is configured as a wedge in FIG. 9 a, as a hemisphere in FIG.9 b and as a cuboid element in FIG. 9 c. Of course, virtually any othergeometrical shapes may be envisaged for producing the recess 16 and theholding element 15.

The type of loading of the connection represented can be adjusted withinwide limits through the shaping of the holding element and the recess.In particular the distribution of compressive, tensile and shear loadsin the filler material 17, which are due to the weight force of theoptical element, may be distributed in an optimized way.

FIG. 10 shows a variant in which the solder is embodied as fillermaterial 17, which is arranged as a spherical preform 4 in the region ofthe recess 16 and the holding element 15 and is melted by the focusedlaser beam 9. When the preform 4 melts, it fills the region between theoptical element 1 embodied as a lens and the holding element 15, andtherefore provides a firm material-fit connection after it hassolidified. The adhesion of the solder to the material of the opticalelement 1 is improved by a metallization layer 6 being appliedbeforehand on the cylinder edge of the optical element 1 embodied as alens. In addition or as an alternative, the holding element 15 may alsobe provided with a metallization 6 in the region of the recess 16. Forthe layer system of the metallization, Cr, W/Ti, Al may be envisaged asadhesion promoters, Ni, Pt, Cr, Ti as a solderable layer and Au, Pt asan oxidation protection layer. Owing to its low oxidizability, the layersequence Cr/Ni/Au represents an advantageous selection. The layer systemmay in particular be produced via ion-assisted vacuum deposition (IAD,with electron beam evaporator—EBE); electrochemical coating methods mayalso be used.

The method described above is suitable in particular for initiallyconnecting the holding elements 15 firmly to the optical element 1. In asubsequent step, the holding elements 15 may then be connected to theframe 2.

FIG. 11 shows an optical element 1 embodied as a lens, in the installedstate in a frame 2. The optical element 1 embodied as a lens is firmlyconnected through the connection material 17 to the holding element 15,which is in turn firmly connected to the frame 2 through the for examplesoldered or welded frame connection position 18. The holding element 15may in particular be introduced stress-free into the frame connectionposition 18 if a small gap remains between the holding element 15 andthe frame connection position 18, which can subsequently be connectedfirmly to the frame 2 in a subsequent step via a welding or solderingmethod. The use of a soldering method can be desirable because thisprovides the opportunity to desolder the optical element 1 together withthe holding elements 15 from the frame in a straightforward way at alater time.

FIG. 12 shows various framing concepts in FIGS. 12 a, 12 b, 12 c and 12d, which can be produced by the method described above. FIG. 12 adescribes an option in which the optical element 1 is only connectedpointwise to the frame 2 through the holding element 15. Optionally, theholding elements 15 may also enclose the optical element 1quasi-annularly along its entire circumference, so that particularlysecure mounting of the optical element 1 in the frame 2 is ensured. Thisoption is represented in FIG. 12 b. Compromises between the two variantsof FIGS. 12 a and 12 b are shown in FIGS. 12 c and 12 d; in these cases,the holding elements 15 do not fully encircle the optical element 1 onthe sides facing it, rather they are produced as segments of differingextent. FIG. 12 c shows a solution with two segments, while FIG. 12 drepresents a variant of the disclosure in which four segments are used,each of which is held in the frame 2 by two holding elements 15.

FIG. 13 represents a projection exposure apparatus semiconductorlithography 31, in which the disclosure is employed. The apparatus isused to expose structures on a substrate which is coated withphotosensitive materials, which in general predominantly consists ofsilicon and is referred to as a wafer 32, for the production ofsemiconductor components, for example computer chips.

The projection exposure apparatus 31 consists essentially of anillumination instrument 33, an instrument 34 for receiving and exactlypositioning a mask provided with a structure, i.e. a so-called reticle35, by which the future structures on the wafer 32 are determined, aninstrument 36 for holding, moving and exactly positioning this wafer 32per se and an imaging instrument, i.e. a projection objective 37, havinga plurality of optical elements 1 which are mounted via frames 2 in anobjective housing 40 of the projection objective 37. The elements 1framed may be arranged at any position in the projection objective 37;arrangement as a first or last optical element of the projectionobjective is suitable, since the most stringent desired properties fortightness and mechanical stability in respect of the framing techniqueexist at these positions. Optionally, the optical element 1 framed mayalso be arranged in the illumination instrument 33. In the projectionexposure apparatus represented by way of example, all the methodspresented above for arranging optical elements in frames may beemployed, as well as all the described arrangements and devices.

The basic functional principle of the apparatus represented is that thestructures introduced into the reticle 35 are imaged onto the wafer 32;the imaging is generally carried out on a reducing scale.

After exposure has been carried out, the wafer 32 is moved further inthe arrow direction so that a multiplicity of individual fields areexposed on the same wafer 32, in each case with the structure dictatedby the reticle 35. Owing to the stepwise forward movement of the wafer32 in the projection exposure apparatus 31, the latter is often alsoreferred to as a stepper.

The illumination instrument 33 provides a projection beam 41 which isdesired for imaging the reticle 35 on the wafer 32, for example light orsimilar electromagnetic radiation. A laser or the like may be used as asource of this radiation. The radiation is shaped by optical elements inthe illumination instrument 33 so that the projection beam 41 has thedesired properties in respect of diameter, polarization, wavefront shapeand the like when it strikes the reticle 35.

Via the beams 41, an image of the reticle 35 is generated andtransferred, correspondingly reduced by the projection objective 37,onto the wafer 32 as already explained above. The projection objective37 has a multiplicity of individual refractive, diffractive and/orreflective optical elements 38, for example lenses, mirrors, prisms,cover plates and the like.

The variants described above allow reversible, stress-free andmaterial-fit connection of optical elements and frames. In particular,the described connections have a high creep stability; with them, it ispossible to achieve position changes <100 nm and surface deformationchanges <1 nm over several years. Fixing strengths in the range of >30N/mm² can furthermore be achieved for optical elements with a widevariety of geometries having a diameter >200 mm. The properties can inparticular be ensured even when there is a difference in the rangeof >10% between the thermal expansion coefficients of the frame and theoptical element. The disclosure furthermore makes it possible for thestresses due to the material-fit connection in the optical element notto exceed a value of 1 MPa.

1. A method, comprising: locally melting and subsequently solidifying aconnection medium on at least one connection position between an opticalelement and a frame so that the optical element is connected to theframe only through the connection medium, wherein the optical element isin the frame, and the optical element and the frame are in mechanicalcontact with one another via at least three connection positions throughnon-molten regions of the connection medium during each instant of thelocal melting and subsequent solidification.
 2. The method as claimed inclaim 1, wherein: the optical element and the frame are connectedtogether on at least four connection positions; and the methodcomprises: providing energy to at least three of the connectionpositions so that a non-molten region of the connection medium ispresent while providing the energy; and subsequently connecting theoptical element and the frame by fully melting the connection medium atthe connection positions.
 3. The method as claimed in claim 2, whereinthe optical element is a circular optical element, on the circumferenceof which the at least three connection positions are arranged at anangular spacing of about 120°.
 4. The method as claimed in one of claim2, wherein providing energy comprises irradiating the at least threeconnection positions with a laser beam.
 5. The method as claimed inclaim 5, wherein the laser has a power of about 100 watts for about 50milliseconds with a laser spot size of about 1 mm².
 6. The method asclaimed in claim 1, wherein, prior to being locally melting andsubsequently solidified, the connection medium comprises a solderpreform with a thickness of about 50 μm to 100 μm and lateral dimensionsof about 5×5 mm.
 7. The method as claimed in claim 1, wherein theconnection medium is vapor deposited, sputtered or electrolyticallyapplied onto the optical element and/or the frame.
 8. The method asclaimed in claim 1, wherein the connection medium comprises a soldercomprising tin, lead, bismuth, indium, antimony, gold, silver and/orpalladium.
 9. The method as claimed in claim 2, wherein fully meltingthe connection medium comprises a controlled soldering process whilemonitoring the temperature of the connection position.
 10. The method asclaimed in claim 1, wherein the optical element is a lens or a mirror ina projection exposure apparatus for semiconductor lithography.
 11. Aprojection exposure apparatus, comprising: a frame; an optical elementarranged in the frame; and a solder that connects the optical element tothe frame, wherein the optical element is in mechanical contact with theframe only through the solder, and the projection exposure apparatus isa semiconductor lithography projection exposure apparatus.
 12. Theprojection exposure apparatus as claimed in claim 11, wherein theoptical element is a lens.
 13. The projection exposure apparatus asclaimed in claim 12, wherein the lens has an edge thickness <1 cm. 14.The projection exposure apparatus as claimed in claim 11, wherein theoptical element is a mechanically manipulable lens.
 15. The projectionexposure apparatus as claimed in claim 11, wherein the projectionexposure apparatus comprises a projection objective, and the opticalelement is the first or last optical element of the projection objectivein a beam direction.
 16. The projection exposure apparatus as claimed inclaim 11, wherein the projection exposure apparatus is an EUV projectionexposure apparatus, and the optical element is a non-manipulated mirror.17. The projection exposure apparatus as claimed in claim 11, whereinthe solder comprises tin, lead, bismuth, indium, antimony, gold, silverand/or palladium.
 18. A method, comprising: using a solder to connect anoptical element to a frame on at least one connection point; and whileconnecting the optical element to the frame, using a bearing element ofthe frame to mutually position the optical element and the frame toavoid relative movements of the optical element and the frame, whereinthe optical component is a component of a projection exposure apparatusfor semiconductor, and the solder comprises tin, lead, bismuth, indium,antimony, gold, silver and/or palladium.
 19. The method as claimed inclaim 18, wherein the bearing element is an integral element of theframe.
 20. The method as claimed in claim 18, wherein the bearingelement is releasable from the frame.
 21. The method as claimed in claim18, wherein the optical element rests on the bearing element at leastthree points.
 22. The method as claimed in claim 18, wherein the solderis applied as a layer on the frame and/or on the optical element beforeconnecting.
 23. The method as claimed in claim 18, wherein, prior tousing the solder to connect the optical element and the frame, thesolder is a preform on the frame and/or on the optical element.
 24. Themethod as claimed in claim 23, wherein the geometrical shape of thepreform can be utilized to align the optical element on the frame. 25.The method as claimed in claim 23, wherein the preform is melted onlylocally during the soldering process.
 26. The method as claimed in claim18, wherein the shape of the frame follows the shape of the opticalelement in the region of the at least one connection position, and thesolder has an approximately constant thickness.
 27. The method asclaimed in claim 18, wherein the solderable system is applied onto theoptical element before connecting the optical element and the frame. 28.The method as claimed in claim 27, wherein the solderable systemcomprises openings.
 29. The method as claimed in claim 27, wherein thesolderable system is not bounded by sharp edges on the optical element.30. The method as claimed in claim 18, wherein the solder is heated by alaser beam.
 31. The method as claimed in claim 30, wherein the laserbeam passes through the optical element.
 32. The method as claimed inclaim 30, wherein a solderable system is applied onto the opticalelement before connecting the optical element and the frame, the laserbeam is reflected at least partially by the solderable system, and thereflected fraction heats the solder.
 33. The method as claimed in claim30, wherein the frame is heated by an additional laser beam in theregion of the connection position.
 34. The method as claimed in claim33, wherein the first and additional laser beams are formed from thesame laser source.
 35. The method as claimed in claim 33, wherein theadditional laser beam is formed from an additional laser source.
 36. Themethod as claimed in claim 18, wherein the bearing element is in contactwith the optical element in the region of an optically active surface ofthe optical element, and the connection position lies on an opticallyinactive surface of the optical element.